Step sinusoidal voltage controlling method for hid, flourescent and incandescent light dimming applications

ABSTRACT

A step voltage controlling device and method for High Intensity Discharge (HID) light dimming applications includes an autotransformer, voltage sensing block, current sensing block, pulse forming block, microprocessor control unit (MCU) block and plural three switch blocks which allows an instantaneous variation of the voltage applied to the load which is controlled to be synchronized with the load current zero crossing point.

FIELD OF THE INVENTION

This disclosure relates to alternating current voltage controllers, moreparticularly to controllers for varying the AC voltage applied to alarge variety of lighting loads, including, but not limited to all typesof gas discharge lamps as well as incandescent lamps. This applicationclaims priority under the Paris Convention to U.S. ProvisionalApplication for Pat. Ser. No. 60/481,804 filed on Dec. 17, 2004, theentire contents of which are incorporated herein by reference.

BACKGROUND

A number of electronic high frequency switching AC voltage controllershave been devised for delivering a variable sinusoidal output voltage.U.S. Pat. No. 5,018,058 discloses how to obtain a pure sinusoidal outputvoltage waveform from the AC line voltage using switchmode electroniccircuitry. Furthermore, U.S. Pat. Nos. 5,500,575 and 5,714,847 showother methods of obtaining a variable AC output voltage having asinusoidal waveform. Although existing electronic high frequencyswitchmode AC voltage controller have several major advantages, such asa small size and weight and a relatively low manufacturing cost, theirmain disadvantage is the lack of capacity to correct the output voltagewaveform when a heavy inductive and non-linear load, such as anelectromagnetic ballast and a HID lamp are present. The result is arelatively large distortion of the output voltage that decreases theoverall system efficiency.

BRIEF SUMMARY

The above described current/voltage distortions are at least reduced, ifnot eliminated by various embodiments of the invention by continuouslymagnetically coupling a version of the AC line voltage to the outputvoltage.

Although adjusting the output voltage in steps may be generally knownfor some applications, there are a number of loads, such as a HighPressure Sodium (HPS) Ballast, that do not accept this type of voltagecontrol. Indeed, a HPS lamp will simply extinguish if the appliedvoltage sharply decreases. Embodiments of the invention described hereineliminates this inconvenience by providing a novel voltage step controlalgorithm, among other features.

In various embodiments of the invention, a step sinusoidal voltagecontrolling device and method for HID, fluorescent and incandescentlight dimming applications includes use of a multiple tap step downautotransformer, multiple switch block, voltage sensing block, currentsensing block, pulse forming block, and microprocessor control unit(MCU) block. In this context, step voltage-controlling means controllingan instantaneous variation of the voltage applied to the load that issynchronized with the load current zero crossing point.

DESCRIPTION OF THE DRAWINGS

FIG. 1 represents the basic block diagram of the step sinusoidal voltagecontrolling device for HID, fluorescent and incandescent light dimmingapplications, for example;

FIG. 2 represents the internal structure of the voltage sensing block 1,current sensing block 2 and pulse forming block 3;

FIG. 3 describes the internal structure of the three identical switchblocks 5, 6 and 7, respectively;

FIG. 4 describes the automatic elimination of the effect caused by thetemperature coefficient of comparators 301 and/or 302 contained by thepulse forming block 3;

FIG. 5 describes the how the MCU block 4 automatically determines theideal moment for a step voltage change based on the voltage/currenttrend evaluation; and

FIG. 6 shows in detail die step voltage variation algorithm for avoidingany load anomaly, such as a HPS lamp being extinguished by theinstantaneous change of the applied voltage.

DETAILED DESCRIPTION

An embodiment of the invention is illustrated in FIG. 1, and this deviceincludes a multiple tap step-down autotransformer 8, voltage sensingblock 1, current sensing block 2, pulse forming block 3, MCU block 4 andswitch blocks 5, 6 and 7. As this embodiment is energized, MCU block 4generates a command signal on line 60 for turning on switch block 5.This means that the step voltage controlling device always starts atfull output voltage measured between lines 11 and 21, a voltage that isequal to the voltage measured between lines 10 and 20. It is generallyknown that a HID ballast must be first energized at its nominal voltage,and it must continue to operate at the nominal voltage until it reachesits normal operating parameters. This period of time when the HIDballast must first operate at nominal voltage is known as “lamp warm-uptime”.

FIG. 5 indicates the voltage/current phase angle for a HID ballast. Whenthe ballast is first energized, the voltage current phase angle has arelatively high negative value. As the lamp warms-up, the phase angledecreases. When the phase angle stabilizes, it means that the ballasthas reached its normal operating parameters, and therefore the ballastis capable to support an instantaneous variation of the applied voltageon line 11, from the level on line 10 to the level on line 30. MCU block4 will remove the command signal on line 60, therefore turning off theswitch block 5. The sequence to turn off switch block 5 is associatedwith a command signal generated on line 61 by MCU block 4 for turning onswitch block 6. After this instantaneous applied voltage variation, thevoltage/current phase angle sharply deteriorates; it drops at a low andstable value as the lamp is re-adjusting its temperature, as determinedby the lower level of voltage on line 30. After the phase angle becomesstable, the ballast is, again, capable to support a new instantaneousvariation of the applied voltage, from the level on line 30 to the levelon line 40.

MCU block 4 will remove the command signal on line 61, therefore turningoff the switch block 6. Turning off the switch block 6 sequence isassociated with a command signal generated on line 62 by the MCU block 4for turning on switch block 7.

FIG. 2 describes the voltage sensing block 1, current sensing block 2and pulse forming block 3. Resistors 101 and 102 perform the voltagedivider function for delivering to pulse forming block 3 of a lowamplitude signal via line 50. The current sensing block 2 includescurrent sensing resistor 201 but, alternatively, a current transformermay be-also used, without departing from the spirit of the inventiveconcept.

A signal equivalent to the ballast current on lines 10, 11, 21 and 20 isapplied to the pulse forming block 3 by the current sensing block 2 vialine 22. Pulse forming block 3 contains the two comparators 301 and 302for converting the positive half cycles of signals on lines 20 and 22 insquare wave pulses to be delivered to the MCU block 4 via lines 51 and52.

FIG. 3 describes each of the switch blocks 5, 6 or 7. It consists of apower triac Tr, a triac conventional snubber network consisting ofresistor R and capacitor C, and a metal oxide varistor MOV for absorbingany possible damaging high voltage transients. For safe interface withthe MCU block 4 via line 60, 61 or 62, the triac Tr gate is energized byan optically isolated triac driver Tr Drv. There are many availabletriac gate driver opto-couplers available on the market; their use isknown in the art, and is not described herein.

FIG. 4 shows the automatic compensation of an eventual input offsetvoltage drift caused by its inherent temperature coefficient. Graph (1)shows the AC line voltage AC IN on lines 10 and 20 equivalent signal online 50. Graph (2) shows the ballast current on lines 10, 11, 21 and 20equivalent signal on line 22. It is assumed that neither of comparators301 and 302 of the pulse forming block 3 have any input voltage offsets.Graph (3) and graph (4) show the pulses on lines 51 and 52. There aretwo methods to determine the phase of the pulses on lines 51 and 52. Thefirst method is to compare the time interval between the fronts (risingfront) of pulses on lines 51 and 52, while the second method is tocompare the time interval between the middle of pulses on lines 51 and52. The middle of each pulse on lines 51 and 52 is determined by the MCUblock 4 by dividing in half the P and R number of externally generatedhigh frequency pulses counted during the duration of each pulse on lines51 and 52. The middle so determined of the pulses on lines 51 and 52should be named “medians”. Indeed, as all graphs (1), (2), (3) and (4)show, in the absence of any input voltage offset of comparators 301 and302 contained by the pulse forming block 3, both methods are error free.However, as graphs (5), (6), (7) and (8) indicate, when one of thecomparators 301 and 302 contained by the pulse forming block 3 isexperiencing a non-zero input offset voltage, the only method thateliminates any voltage/current phase angle measurement requirement iswhen measuring the time interval between the medians of pulses on lines51 and 52, rather than when measuring the time interval between thefronts of these pulses on lines 51 and 52.

FIG. 6 describes in detail the mechanism of instantaneous voltagevariation applied to a ballast for avoiding extinguishing the lamp.Assuming that all conditions described in FIG. 5 for the instantaneousvoltage changes are met, the dimming sequence is now described. At thecurrent median described in FIG. 4, MCU block 4 is removing the commandsignal on line 60. It is known that a triac continues to conduct currentuntil the current value becomes zero; this is happening at the nextcurrent zero crossing. Based on this well-known triac property, thecurrent via switch block 5, therefore the current through the ballast onlines 10, 11, 21 and 20 becomes zero. When the ballast current becomeszero, the MCU block 4 instantaneously generates a command signal on line61, thereafter turning on switch block 6. As a result, the ballastcurrent through lines 10, 11, 21 and 20 continues to flow, following anon-discontinued sinusoidal waveform. Next, MCU block 4 decides to oncemore reduce the ballast-applied voltage. It will then remove the commandsignal on line 61 at the ballast current median. At the next ballastcurrent zero crossing, switch block 6 turns off. During this ballastcurrent zero crossing, MCU block 4 generates a command signal on line62, turning on switch block 7.

The ballast-applied voltage could suffer instantaneous variations inreverse order, in two increments, or by simply applying the voltage online 10 in one step, if the algorithms described in FIG. 5 and FIG. 6are observed by MCU block 4.

1. A step voltage controlling device, comprising: a multiple tapstep-down autotransformer connected to an AC input voltage appliedacross first and second AC voltage input terminals; a voltage sensingblock connected across the first and second AC voltage input terminals;a current sensing block connected between the first AC voltage inputterminal and a first output terminal; a pulse forming block operativelyconnected to receive signals from each of the voltage sensing block andthe current sensing block and to provide at least two pulsed outputsignals responsive to the received signals; a controller block whichreceives the at least two pulsed output signals from the pulse formingblock and which responsively provides a plurality of command signals; aplurality of switch blocks each receiving an associated one of theplurality of command signals from the controller block, wherein oneswitch block of the plurality of switch blocks selectively electricallycouples the second AC voltage input terminal and a second outputterminal in response to the associated one of the plurality of commandsignals from the controller block, wherein remaining ones of theplurality of switch blocks each separately and selectively electricallycouples an associated output tap of the multiple tap step-downautotransformer to the second output terminal, wherein, the controllerblock at least initially generates a command signal which turns on theone switch block so as to provide a full output voltage between thefirst and second output terminals, wherein, after a nominal period oftime, the controller block initiates a reduction of an output voltageapplied voltage across the first and second output terminals by removingthe command signal applied to the one switch block so as to turn off theone switch block and to turn on another one of the plurality of switchblocks so as to provide a reduced output voltage between the first andsecond output terminals.
 2. A method for step controlling a voltageapplied to a load, the method comprising: providing a multiple tapstep-down autotransformer; providing multiple voltage switching blockseach coupled electrically to various taps of the multiple tap step-downautotransformer; sensing an input voltage; sensing a load current;forming plural pulsed signals based upon the sensed input voltage andload current; processing the plural pulsed signals and generatingcommand signals for each of the multiple voltage switching blocks;determining a zero-crossing point of the load current; and controllingan instantaneous variation of a voltage applied to the load bysynchronizing with the determined zero crossing point.